The survival of cellular organisms is dependent on the physical properties of water. The freezing point of liquid water sets the lower limit for the survival of most cells, because the formation of ice causes dehydration and osmotic damage to the cell. Organisms which inhabit sub-zero environments have special adaptations which permit the organism to survive. For example, Arctic and Antarctic fish which live in cold seawater have various macromolecular antifreeze polypeptides in the serum of their blood. Such antifreeze polypeptides are a mixture of glycoproteins having a range in relative molecular mass (M.sub.r) from about 2,500 to 34,000 (antifreeze glycoproteins, or "AFGPs") and antifreeze polypeptides (AFPs) with M.sub.r from about 3,300 to 12,000. Ananthanarayanan (1989) Life Chemistry Reports 7:1-32 provides an overview of AFPs and AFGPs. See also DeVries (1983) Annu. Rev. Physiol. 45: 245-260; Davies et al. (1990) FASEB J. 4: 2460-2468 and Warren et al U.S. Pat. No. 5,118,792.
At present, three distinct types of AFPs have been characterized from a variety of cold water fish. See, Davies et al. (1990) FASEB J. 4: 2460-2468; and Griffith and Ewart et al. (1995) Bioteca Adv. 13(3): 375-402, and references therein. Type I AFPs are alanine-rich, ((.alpha.-helical polypeptides, found in many right eye flounders and sculpins. Type II AFPs are enriched with half cystine and are found in sea raven, smelt and herring. Type III AFPs are globular proteins found in several Zoarcoid families including eelpout and wolfish. The AFGPs, which are found in three families of Antarctic fish and polar cods, largely consist of a tripeptide repeat (Ala-Ala-Thr) with a disaccharide attached to the threonyl residue.
Although the different AFPs and AFGPs are structurally distinct, they share the ability to inhibit ice crystal growth by binding to the ice surface. At present, the AFGPs and AFPs have been isolated from serum, and their DNA sequences deduced by cDNA cloning from the liver. All of the proteins described to date are synthesized as larger precursor polypeptides containing the signal peptides, indicating a secretory role for the polypeptides.
AFPs in the liver (liver type AFPs) from the winter flounder, Pleuronectus americanus, has been studied extensively in terms of its protein structure and function, gene organization, gene expression and regulation. The genome of the winter flounder contains multiple copies of liver or serum type AFP genes, most of which are arranged as regular tandem repeats (Scott et al. (1985) Proc. Natl. Acad. Sci. USA. 82: 2613-2617).